This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-075096, Mar. 19, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a signal generator synchronized to an external device having an oscillation frequency divisible by an integer, such as 10 MHz. Furthermore, the present invention relates to an OFDM modulator for conducting frequency conversion on the basis of an OFDM clock (FCLK) and a local frequency (FLO).
The OFDM modulator generates an OFDM signal having a center frequency FCLK by using an OFDM clock (FCLK), converts frequency conversion on the OFDM signal by using a local frequency (FLO), and outputs an OFDMIF signal having a center frequency FIFO. FCLK and FLO need to be synchronized to an external reference signal (FREF) of 10 MHz and thereby stabilized. Therefore, a synchronization circuit is needed for each of oscillators.
A conventional OFDM modulator includes an OFDM modulator section supplied with the OFDM clock (FCLK) from an OFDM clock oscillator, and a local frequency (FLO) oscillator. Furthermore, the conventional OFDM modulator includes a frequency converter section for conducting frequency conversion on signals supplied from them. As a result, an IF signal is outputted. In the conventional OFDM modulator, however, 8.192 MHz is used as the OFDM clock (FCLK) and 45.342 MHz (=8.192 MHz+37.15 MHz) is used as the local signal as an example, in order to achieve synchronization to an external device. Modulation processing and frequency conversion processing are thus conducted.
In the case of the system of terrestrial digital broadcast, however, it is not possible to use the above described divisible frequency such as 8.192 MHz. As for FCLK, 512/63=8.126984 . . . MHz is demanded. Therefore, the local signal for frequency conversion also becomes FLO=FIFO+FCLK=37.15+8.126984 . . . =45.286984 . . . MHz. However, this frequency has a floating component. Therefore, neither of them can be synchronized to the external reference signal of 10 MHz. (Herein, a frequency having a part which is indivisible by an integer, such as 45.286984 . . . MHz is referred to have a floating frequency component.)
In the conventional OFDM modulator, therefore, the demanded OFDM clock (FCLK) of the terrestrial digital broadcast cannot be met. If this frequency FCLK is used as it is, synchronization to the external frequency (10 MHz) is impossible. This results in a problem that an IF signal output having a floating frequency component is outputted.
In not only the OFDM modulator but also a general signal generator, synchronization to the external frequency (10 MHz) is impossible in the case where the clock oscillator includes a floating frequency component. In the same way, therefore, there exists a problem that an IF signal output having a floating frequency component is outputted.
An object of the present invention is to provide a signal generator synchronized to an external device while using a clock oscillator of a frequency which is indivisible by an integer and consequently which has a floating frequency component.
Another object of the present invention is to provide an OFDM modulator for producing an IF signal output having no floating frequency component by using an OFDM clock (FCLK) which is the system of the terrestrial digital broadcast.
In accordance with the present invention, a signal generating apparatus includes: output means for conducting predetermined processing on a given signal, based upon a clock signal having a first frequency (FCLK, f1) including a floating frequency component supplied from a first oscillator, and outputting a first signal having the first frequency; adder means for adding a second signal having a second frequency (FPLO, f2) including no floating frequency component, supplied from a second oscillator (and the clock signal supplied from the first oscillator, and outputting a third signal having a frequency equivalent to sum of the first frequency and the second frequency; and frequency converter means for receiving the first signal outputted from the output means and the third signal outputted from the adder means, conducting frequency conversion based upon the first signal and the third signal, and thereby outputting an output signal having the second frequency including no floating frequency component.
According to the present invention, the above described structure is used. Even in such a signal generator that a clock signal includes a floating frequency component, the second oscillator outputs a second frequency which is divisible by an integer and which does not include a floating frequency component, and an intermediate frequency addition and subsequent frequency conversion processing are conducted. As a result, a finally obtained signal can be made to have the same frequency as the frequency supplied from the second oscillator. According to the present invention, therefore, it becomes possible to provide a signal generator capable of outputting such a signal that a PLL (phase locked loop) can be formed with an external device having a reference oscillator of, for example, 10 MHz.
Furthermore, the present invention can also be applied to a signal generator especially conducting OFDM modulation, i.e., to an OFDM modulator. In this case as well, the same operation effects are brought about.
In other words, subtraction is conducted in a frequency converter circuit 15, between an OFDM clock (FCLK) component supplied from an OFDM modulator section side and an OFDM clock (FCLK) component supplied from a frequency adder 16 side of a frequency converter circuit 15. As a result, a finally obtained IF signal output have the same frequency as that of the local oscillator. Accordingly, a PLL (phase locked loop) can be formed with an external device having a reference oscillator of, for example, 10 MHz. Also as for the OFDM modulator using an OFDM clock oscillator having a frequency of 512/63=8.126984 . . . MHz demanded in the system of terrestrial digital broadcast, therefore, it becomes possible to provide an OFDM modulator which outputs an IF signal having a frequency of, for example, 37.15 MHz according to the frequency of the local oscillator and allowing synchronization to a peripheral reference oscillator.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.